wait up for sb.

　　To wait for sb. to come home at night before you go to bed.

　　所以，如果我们想表达“我晚上会很晚回来，所以不要等我啦先睡吧。”就可以说I'll be back very late so don't wait up for me.

　　cover for sb.

　　这个短语有两个意思：

　　1. 在某人不在的时候帮忙顶替，做他们的工作，比如：

　　I'm covering for Jane while she's on leave.

　　2. 帮某人打掩护，比如说谎啦之类的，以防那位陷入麻烦。比如：

　　I have to go out for a minute—will you cover for me if anyone asks where I am?

　　come to do sth.

　　这个短语呢有种“渐渐”的感觉，有种过程在里面，看他的英文解释：

　　To reach a point where you realize, understand or believe sth. 比方说：

　　In time she came to love him. 最终，她逐渐爱上了他。= He began to grow on her.